1. Field of the Invention
The present invention relates to a concentration measuring apparatus which computes a concentration of a substance to be detected based on an eigenfrequency of the substance to be detected in a mixed solution.
2. Description of the Related Art
Conventionally a number of concentration sensors utilizing a change in sound velocity according to a change in solution concentration are used. In the concentration sensor, the density can be computed by the change in sound velocity because an equation of c=√(K/ρ) holds with respect to a sound velocity c, a density ρ, and volume elastic coefficient K.
More specifically, when the solution concentration is changed, both the density and volume elasticity are changed, which results in the change in sound velocity. Therefore, the solution concentration can be computed from the measured sound velocity by previously finding out a correspondence between the concentration and the sound velocity. However, generally temperature dependence of the volume elasticity is larger than that of the density. That is, because the temperature dependence of the sound velocity is larger, it is necessary that the temperature is strictly managed in measuring the change in sound velocity.
For example, in the case where the change in concentration within ±15% of a 3 wt % methanol solution at liquid temperature of 40° C. is measured by the sound velocity type concentration sensor, it is necessary the liquid temperature is managed with an accuracy not more than ±0.1° C. That is, a device for strictly managing the liquid temperature is required, which results in a problem that the concentration sensor cannot be miniaturized.
On the other hand, a technology in which the concentration is computed from the eigenfrequency of an object has been proposed as the technology in which the sound velocity dependence is smaller than that of the sound velocity type concentration sensor in computing the concentration.
For example, in the technology disclosed in Japanese Patent Application Laid-Open No. H6-18394, the concentration of the substance to be detected in the mixed solution is determined by determining the eigenfrequency of a quartz oscillator when the quartz oscillator dipped in the mixed solution is oscillated.
However, in the technology disclosed in Japanese Patent Application Laid-Open No. H6-18394, there is a problem that a flowable area such as the mixed solution which is of the detection target has an influence on accuracy of the computed concentration.